(i) Field of the Invention
The present invention relates to a digital modulator for use in a multi-level quadrature amplitude modulation (QAM) type digital radio communication system, especially to a digital modulator which can automatically set a modulation parameter suitable for a change of a system transmission parameter based on a control signal from the outside.
(ii) Description of the Related Art
A modulator whose modulation parameter is controlled at the time of malfunction or operation is heretofore known. For example, in a known technique, outside control data including the modulation parameter is entered into a modulator and controlled in a software manner. Examples of the outside control data include control data concerning the bit number of two orthogonal data strings or a cut-off frequency of a waveform shaping filter. In accordance with the control data an orthogonal data modulating section of the modulator and the operation frequency of the waveform shaping filter are controlled to automatically change the modulation parameter.
In the prior art described above, the control data is transmitted to the modulator in a software manner when the modulation parameter is changed. Here, when power supply is turned on in the modulator, the control data is set after modulator operation is stabilized. In a transient state from the turning on of the power supply till the setting of the control data, the modulator operation becomes unstable in some case. Furthermore, even when the control data is entered, in a transient state in which the previous modulation parameter is shifted to a new modulation parameter, the modulator operation becomes unstable in some case. In the transient states, if an output spectrum of the modulator is spread beyond a required band which is determined in the system, other adjoining circuits are disadvantageously interfered with. The problem will be described in detail with reference to the drawings.
FIG. 7 shows an RF spectrum waveform when an output frequency of a digital modulator is converted to an RF frequency. The spectrum waveform shown by a solid line in FIG. 7 represents a modulation spectrum for obtaining required characteristics. Its central frequency is represented by f.sub.0, and its bandwidth is represented by f.sub.r. Channels adjoining the modulation spectrum are shown by dotted lines, and a central frequency of each of the channels is apart by .+-.f.sub.r from the central frequency f.sub.0.
When malfunction occurs immediately after the power supply turns on, and incorrect data with a double bandwidth is set, then the bandwidth with the central frequency f.sub.0 is spread to 2f.sub.r. As a result, the modulation spectrum becomes an interference signal with the adjoining channels.
To solve the aforementioned problem of interference with the adjoining channels, for example, a system is disclosed in Japanese Patent Application Laid-open No. 77324/1989 in which adjoining circuits are prevented from being interfered with by lowering an output of a power amplifier. However, in the method, since the output is lowered, an input level of a next-stage transmission device is lowered and an apparatus failure alarm (ALM) is therefore generated. Furthermore, in a next-stage transmitter ALC circuit, full gain is provided. Therefore, the interference with the other adjoining circuits cannot be solved yet.